Recently, a projector is used in a presentation at a conference and etc. or in a home theater in a common household.
Such a projector has a light source and a device (e.g. transmissive liquid crystal display device, digital micro mirror device, and etc.) operated according to image information. The projector projects outgoing beam from the light source onto the device, and expands optical images to be displayed. An example of such a light source is a high-pressure mercury lamp that is closer to a point light source and has high luminance and high color rendering property. The high-pressure mercury lamp has, for example, 200 [mg/cm3] of mercury enclosed therein as a light emitting material, and an arc tube in which a pair of electrodes made of tungsten is disposed.
In general, to activate this high-pressure mercury lamp, a high voltage pulse at 20 [kv] or higher needs to be applied between the pair of electrodes. However, in order to generate such a high voltage pulse, a large scale transformer, a high voltage resistant electronics component or the like needs to be used in the lighting circuit. The use of these components is detrimental for cost-cutting or downsizing of the entirety of the high-pressure discharge lamp system having the lighting circuit.
In response to the above, in order to lower the starting voltage, conventionally, it is applied practically that a conductor made of a metal wire called a proximity conductor or a trigger wire is attached to the arc tube. A variety of innovative ideas of attachment of the conductor are suggested for lowering the starting voltage.
For example, the following is suggested. By disposing a top end of the trigger wire externally of the arc tube, when the lamp is activated, remarkable inequality in the electronic field is formed between the top end and the electrode inside the arc tube, thereby promoting dielectric breakdown between the pair of electrodes, such that the high-pressure pulse of the starting voltage can be decreased to 4-8 [kV] (e.g. Patent Citation 1).
In addition, the following is suggested. The location of a portion of the proximity conductor is limited in a designated area of the sealing part of the arc tube, and the proximity conductor is wound such that a closed loop is not formed. The high-pressure pulse of the starting voltage can be decreased to 8 [kV] or less (e.g. Patent Citation 2).
However, when the high-pressure discharge lamp system is designed as mentioned above in order to satisfy the demands for downsizing the entirety of the lamp and cost-cutting, the upper limit of the high-pressure pulse that can be generated by the lighting circuit is approximately 4.0 [kV]. Then, when a margin is taken into consideration for practical use, the starting voltage of the high-pressure mercury lamp is demanded to be 3.0 [kV]-3.5 [kV], inclusive. It is considered that merely adopting the trigger wire (proximity conductor) and devising the attachment thereof, described as above, cannot fully satisfy these demands. Hence, in order to meet such demands, in addition to the adoption of the trigger wire, a radioactive material, such as Krypton 85 (85Kr), is enclosed within the arc tube.    Patent Citation 1: JP No. 2003-317663    Patent Citation 2: WO No. 2004-090934